The fossil fuels today mainly burnt in particular in industrial uses, for example in power stations, large-scale furnace plants, refuse incinerating plants, etc., contain sulphur in elemental and bonded form. Crude oil contains between about 0.2 and 7% by weight sulphur. In the distillation and refining of crude oil, the sulphur concentration is highest in the heavy fractions and in the residues. Heavy fuel oil can contain between 1.5 and 7% by weight sulphur with, between 10 and 22% of the sulphur contained therein being bonded to asphaltenes.
Enormous efforts have already been made to limit the sulphur dioxide content of the air, which originates to a considerable extent from combustion plants operated with fossil fuels. However, these efforts were mainly directed towards desulphurizing the flue gases formed in the combustion. Although today flue gas desulphurization apparatuses operating with good efficiency exist, they have however the disadvantage of being extremely expensive. The principle employed, i.e. first burning the sulphur to form sulphur dioxide which is extremely dangerous to the environment and then tediously the pollutants thus generated, is fundamentally very unsatisfactory. Compared therewith, it appears more reasonable to try to bond the sulphur before burning the fuels so that the gaseous pollutants do not form in the first place, or at least try to minimize as far as possible the amount of gaseous pollutants, in particular sulphur dioxide.
In his article "Experiments On Desulphurization Before and After the Burner for Reducing the SO.sub.2 Output" (Mitt. der VGB, no. 83, p. 74-82, April 1963), K. Wickert gives an excellent summary of the desulphurization methods hitherto employed. However, the method described therein for desulphurization prior to the burner also operates in the gas phase, i.e. is effectively a gas purifying method which requires expensive apparatus and a large amount of space. In this known desulphurization prior to the burner (cf. loc. cit. p. 80-82), the fuel, in the specific case residual oils, is gasified by a partial burning at 1000.degree. to 1200.degree. C. The hot combustion gases are then desulphurized with a solid basic additive, that is CaO or CaCO.sub.3, to form CaS, thereafter dedusted and then supplied to a gas burner. However, a problem with this oil gasification by partial burning is the carbon black which forms. For this reason, CaO and MgO are used as desooting catalysts which react with the water vapour forming in the partial combustion step carbon monoxide and hydrogen. Of course, the CO must not be allowed to enter the environment any more than the SO.sub.2, and for this reason it must therefore be washed out of the flue gases again or oxidized to CO.sub.2.
This known desulphurization method is too complicated and too expensive for the conversion of old combustion plants fired predominantly with heavy fuel, and the installation of the desulphurization and dedusting apparatus necessary for this purpose is frequently not possible for the simple reason that no space is available.